When same Url file, different target path, does not update multiple

[kzu]

If the same upstream URL is used to target more than one file,
we currently just process it once and don't properly update the
other copies in other target folders.

We should instead index by file path and URI when skipping
duplicate processing.

NOTE: this would be a bit suboptimal since we'd still fetch the URL
multiple times, but we'd be correct. And it's a bit of a corner case
for now, not worth additional optimization for that for now, pending
user feedback.